Which of the following descriptions about URPF (Unicast Reverse Path Forwarding) is correct?

Question

Options

• AAfter deploying strict mode URPF, you can also deploy it at the same time to allow matching the default routing mode.
• BStrict mode URPF is often difficult to deploy in practical use because it is too restrictive.
• CIf you deploy URPF in loose mode, you need to check the default route, you need to check whether the interface matches
• DIf loose mode URPF is deployed, as long as the packet can match the detailed route, the P address can be deemed legitimate and the packet can be forwarded.

• A
  7% (1)
• B
  86% (12)
• D
  7% (1)

Explanation

Option B is correct because strict mode URPF requires that a packet's source IP address has a return route pointing back out the same interface the packet arrived on. In real networks, asymmetric routing is extremely common - traffic legitimately arrives on one interface but the best return path exits a different interface. Strict mode drops these valid packets, making it nearly undeployable in multi-homed or ISP environments without causing significant disruption.

Why the distractors are wrong:

• A is incorrect because strict mode and loose mode cannot be deployed simultaneously on the same interface - they are mutually exclusive modes. The "allow-default" option in strict mode is a separate add-on, not a concurrent mode deployment.
• C is incorrect because loose mode does not check whether the interface matches - that's the defining characteristic of strict mode. Loose mode only verifies that a route to the source IP exists anywhere in the routing table.
• D is partially misleading: loose mode doesn't restrict checks to only "detailed routes." By default, loose mode may or may not check the default route depending on the platform/config, and matching any route entry (not just specific ones) is the actual criterion.

Memory tip: Think of the modes as bouncers - strict checks both your ID and which door you walked through, while loose only checks if your ID is real at all. Strict is so picky about the door that real guests often get turned away - that's why it's impractical.

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